Field of the Invention
The present invention generally relates to isolators and, more particularly, to optical isolators.
Description of the Related Art
Isolators are important components for photonic systems and for protection of communications and sensor systems. Due to the strong push to develop high performance, low power photonic integrated circuits (PIC), there is a pressing need for footprint reduction and silicon integration in optical isolators. These desired isolator characteristics must also be combined with optimal isolation performance and propagation loss in order to provide necessary performance in PICs.
Contemporary chip scale optical isolator design is based on a Mach-Zehnder Interferometer (MZI) scheme. Typical MZI isolator designs range from 1.5 mm to 5 mm in length, and have an isolation ratio of between 20 dB and 30 dB. A particular MZI device has been shown to provide 20 dB isolation for a bandwidth of 8 nm with a peak isolation ratio close to 30 dB at 1.55 μm, and with a forward insertion loss (excluding coupling losses) of 13 dB.
An MZI isolator generally consists of four waveguides, one input port, one output port, and two waveguides which transport the light from the input to output ports. The last two waveguide elements are coupled to the input and output ports via integrated y-branches or multimode interference devices, which add to the total length, and overall footprint of the device. These beam splitters must be properly balanced to achieve a 50/50 split between the two arms of the interferometer. This can lead to coupling and bending losses associated with coupling light into the MZI and its transport from input to output. The device is also generally covered by a magnetic cladding layer which induces the nonreciprocal phase shift responsible for isolation.
Accordingly, there is a need in the art for an isolator with a simpler design, higher isolation performance, lower forward loss, and smaller footprint.